1. Field of the Invention
This invention relates to methods and apparatus for continuous casting and hot-rolling of metal to produce metal strip, using a continuous production line. The invention is primarily applicable to steel production, though not restricted thereto, and will be described below mainly in relation to steel.
2. Description of the Prior Art
The continuous operation of a continuous casting and hot-rolling facility can save energy and labor to a great extent as compared with conventional separate factories for steelworks and rolling-mill. Such continuously operated facilities have been increasing in number in recent years.
Merits of the continuous operation include decrease of energy consumption, since a slab temperature decrease is made as little as possible to minimize re-heating, and labor saving as no transport of the slab from the steelworks to the rolling-mill is needed.
To make continuous the operations of the continuous casting process and the hot-rolling process, it is desired that the combined facility should be optimized in total with respect to the material to be produced, amount of production, method of obtaining a raw material, facility cost, running cost, energy consumption, and manufacturing and labor costs.
Methods of making continuous the operations of the continuous casting process and the hot-rolling process can be roughly classified into two: a method of rolling a slab a few times with use of roughers before the slab is fed into a finishing rolling mill train which it passes once to roll to a desired production thickness; and a method of feeding a slab into a finishing rolling mill train without rougher, to pass through the mill train only once to roll to a desired production thickness.
However, if the slab is too thick, there are limits on these methods. There can be no realistic facility that can feed a slab, for example of 200 mm thickness into the finishing rolling mill train to pass through the mill train once to roll, for example to 1.6 mm thickness because of requirements for keeping the rolling temperature high and increasing the rolling speed. Thus, it is necessary to install the roughers.
On the other hand, if the slab is thin, the roughers can be omitted, which can lower the facility cost and the production cost. As an example, JP-A-63-132703 discloses a method of making continuous the operations of the continuous casting process for casting the thin slab and the hot-rolling process without rougher. This document proposes a compact facility of casting a relatively thin slab for a medium-scale production of around a million tons per year. It disclosed that up to three or four rolling stands can continuously roll the slab. A first rolling stand has large-diameter working rolls to roll the slab, and a later rolling stand has working rolls and driven back-up rolls, and the slab is intermediately stored between the continuous casting process and the hot-rolling process.
However, in practice, as discussed more below, it is difficult to cast by the continuous casting process to produce a slab of around 50 mm in the slab thickness. Therefore, the facility disclosed in JP-A-63-132703 cannot satisfy the need that the strip required as a final product of the hot-rolling has to be not thicker than 2.0 mm.
The company SMS Schloemann-Siemag AG has published a proposal for a continuous casting and hot-rolling facility called CSP (compact strip production). The casting mould has a funnel shape, unlike the conventional parallel-sided mould. The aim of the funnel-shaped mould is to achieve a cast slab thickness as low as 50 mm. The funnel shape is necessary in order to accommodate the submerged casting nozzle in the mould. Since the solidified metal shell is deformed in such a funnel shape mould, its use brings in an extra requirement to ensure that stability of the casting process is maintained, and that break-out of molten metal does not occur.
In the CSP facility, furthermore, a soaking furnace of rather great length is arranged after the shears which cut the cast slab to length, in order to homogenize the slab temperature. One function of the long furnace is to provide for holding of the slabs at high temperatures, in the event of a temporary stoppage of the rolling mill train. This can be achieved by normally moving the slabs at a speed higher than the casting speed, so that there are gaps between the slabs which can close again if the rolling mill train stops. However the use of a long furnace, which is heated by combustion of fuel, makes the overall length of the facility large.
An alternative solution to these problems has been adopted in the mill installed by Mannesmann-Demag and Arvedi in Cremona, described in MPT-Metallurgical Plant and Technology, No. 2/1992. In this, the slab is cast at 60 mm thickness, and reduced in several roughing mill stands to 15 mm thickness. Because of the cooling occurring in this thickness reduction, a long induction heating stage follows, before the slab is coiled on a coiler mounted in a thermally insulated chamber. The coiler can hold two coils, and one coil is unwound into the hot-rolling train. This process requires a relatively large amount of energy to re-heat the slab after the roughing mill stands, and care is necessary to ensure that the steel does not cool to the A.sub.3 temperature before the reheating. It is not possible to adapt the process by reducing the amount of thickness reduction before reheating, because the coiler cannot coil thicker slabs.
The following prior art documents are discussed below, in context: U.S. Pat. No. 5,133,205, JP-A-58-20301 and JP-A-61-176401.